The present invention relates to servohydraulic actuators. More particularly, the present invention pertains to a piezoelectric direct driven servovalve capable of very high bandwidth operation.
For many high load systems hydraulic actuators have remained a necessary and desirable means of device actuation. The control of these actuators is commonly effected through the application of various types of servovalves. In most servovalves a mechanical or electrical signal is utilized to direct the position of a valve spool within a valve housing. The position of the valve spool within the housing determines the flow path(s) between valve ports to direct flow to the ports of a hydraulic actuator thus determining the direction of force application and motion of the actuator.
The need for higher frequency, more precise control of structures and devices has led to ever improving servovalves and systems. Servovalves have evolved from relatively slow acting two stage devices where the first stage is comprised of a pilot valve. The pilot valve is controlled by some low power means such as human force input or a low power electrical signal and shuttles a pressurized fluid or gas supply in a pilot system which thereby displaces a valve spool in a larger flow, power control valve. The power control valve in turn determines the flow direction to a high force, large displacement actuator. The compliance and inertia in the hardware used for pilot valve operation and the compliance and fluid inertia in the pilot fluid system combine to significantly reduce the frequency response of the power control valve to the original system command input.
The need for faster acting systems has led to the development of various single-stage control valves where a single, directly controlled system or device develops the force necessary to shuttle the valve spool in the flow control valve governing the high force actuator.
Substantial increases in control speed and available system bandwidth have been gained through the application of electromagnetic motors. With electrical amplification of the command signal, an electronic amplifier and electromagnetic motor or actuator directly control the position of the servovalve spool. The force available from the electromagnetic device is utilized to directly drive and position the spool, thus supplanting the pilot valve and pilot pressure system formerly used for power valve actuation.
An example of such a system is disclosed in U.S. Pat. No. 5,040,568. In that patent the rotary motion of an electromagnetic motor is applied directly to the spool of a power valve to effect valve control in a directly driven, single-stage manner. The size of the power valve and the magnitude of the power system pressure being controlled determine the amount of valve spool force necessary for valve control, which thereby determines the size of the electromagnetic motor necessary for spool positioning. A significant disadvantage of the use of electromagnetic motors for servovalve actuation is the relatively weak force and energy density of these drives which limits the size and bandwidth of the servovalve. As the electromagnetic motor size increases, performance frequency drops and system control bandwidth becomes limited by the inherent characteristics and performance capabilities of the motor.
In an alternative approach for gaining system control bandwidth, smart materials, such as piezoelectric, electrostrictive, and magnetostrictive materials, have been employed in valve control systems. With smart materials large forces can be generated for very small distances and controlled at very high frequencies.
In U.S. Pat. No. 5,148,735 a piezoelectric or magnetostrictive element operates as the actuation means to valves controlling the fluid in a pilot fluid system. In this approach the smart material elements in combination with the pilot fluid controlling valves act as a pilot valve to a pilot system so to directly control the fluid pressure applied to the ends of the power valve spool. The approach maintains the compliance and inertia of the pilot system, however, which reduces the bandwidth capability inherent to the smart material element.
The use of a piezoelectric element to directly control the position and movement of a valve spool is disclosed in U.S. Pat. No. 6,170,526 B1. In that patent a piezoelectric actuator controls the position of a poppet valve within its valve body. In that patent the poppet valve change in position is equal or comparable to the change in length of the piezoelectric element, which being very small causes the available valve motion to be very small. The small amount of valve movement afforded severely limits the size of the valve and the flow which can be regulated.
There is a need for a high bandwidth servovalve with frequency response substantially beyond that available from electromagnetic actuated valves which is also suitable for substantially larger flow capability than that afforded by the present smart material actuated valves. The invention described herein provides for such a servovalve.
The invention disclosed is a direct driven, smart material actuated servovalve having substantially increased actuation control bandwidth relative to comparably sized flow valves available in the prior art and thereby is suitable for use in moderate to large flow systems as a single-stage servovalve.
The invention incorporates smart materials, with a preferred embodiment employing piezoelectric materials, to directly drive the spool in a power control servovalve. The achieved control bandwidth is two to five times higher than that achieved by comparably sized valves employing electromagnetic drives. The invention also provides for a package that is more compact and lightweight than servovalves of comparable size.
The invention utilizes the high force, high frequency response characteristics of a smart material element and applies this actuation capability directly to the servovalve spool and without an intermediate pilot fluid system. An embodiment of the invention comprises a valve body with a valve spool having a longitudinal axis, a smart material element, and a mechanical lever. The mechanical lever is mounted to a pivot support means and is positioned relative to the smart material element and valve spool such that expansive motion induced into the smart material element enforces motion on the first end of the mechanical lever which motion is thereby amplified by the lever at the second end of the lever. The second end drives the end of the valve spool thus causing the valve spool to shift along its longitudinal axis an amplified distance commensurate with the expansion and contraction of the smart material element.
The positioning of the smart material element relative to the valve spool is maintained in a compact form such that the mechanical lever is kept as small and stiff as possible to minimize the compliance and mass of the lever and thus maintain the high bandwidth control capability of the smart material element.
The invention also serves as a method for effecting high bandwidth control of a servovalve. An embodiment of the method is comprised of inducing a shape change into a smart material element, mechanically amplifying a component of motion of the shape change of the smart material element, applying the mechanically amplified component of motion directly to the end of the valve spool such that the spool is caused to shift to a different valve spool position, and varying the induced shape change in the smart material in a controlled manner such that the valve spool is forced to shift between valve positions in a commensurate controlled manner.